A gas and steam turbine installation has a gas turbine part and a steam turbine part with at least one pressure stage. The working steam of the steam turbine part is generated by one or more waste-heat boilers fed with the waste heat from the gas turbine part. The pressure stage has at least one steam turbine control valve. In this arrangement, the control valve passage cross section is adjusted by means of a control system whose required value is formed by the use of a control parameter, relating to power, of the pressure stage. A control parameter relating to power means that the control system permits control of the power of the pressure stage. In this arrangement, the control parameter relating to power can also be the power of the pressure stage itself. This will be considered in more detail later.
A discrepancy between the instantaneous main system frequency and a required main system frequency is determined in a frequency control system and counteracting control action is taken as compensation for the discrepancy.
Main system operators must guarantee fundamental mains system operational properties. This, in particular, also includes a certain temporal electrical frequency (Europe: 50 Hz) which is stable with respect to the electrical power demanded. Discrepancies from this are only tolerated within certain narrow limits. The frequency stability in the main system is ensured by use of dynamic load/power compensation. For this purpose, substantial reserve power must be available within seconds. The main system operators must be able to offer this reserve power as a service.
In the case of combined gas and steam turbine installations, this reserve power has previously been made available by the gas turbine part of the installation. Combined gas and steam turbine installations are installations in which waste-heat boilers are connected downstream of the gas turbines in order to operate a steam turbine installation. In this arrangement, the exhaust gas temperature of the gas turbines is generally kept constant over a wide power range. In such operation, however, there are limits to the change in the gas turbine power. The load-changing capability is essentially limited by the dynamics of the exhaust gas temperature control and, therefore, by the ability to change the air mass flow through the gas turbine. The steam turbine part of the installation generally follows the power changes of the gas turbines with the substantially more sluggish time response of the waste-heat process. In the case of combined gas and steam turbine installations using frequency control, the change in the power demanded by the main system is currently, therefore, brought about exclusively by the gas turbine part of the installations, because the steam turbine part makes no contribution in the initial seconds. The water/steam circuit, i.e. the steam generation and the steam turbine, is only a passive part of the overall gas and steam turbine installation and acts only as a waste-heat recovery unit.
In consequence, the gas turbine alone must provide the total reserve power in the case of primary control. During the operation of the installation, therefore, the frequency control reserves have to be included in the calculations for the gas turbine alone and the block power in steady-state operation is then reduced by a correspondingly high proportion.